toolchain: fix stampfile dependency

toolchain: fix stampfile dependency

Use the gcc/final/install target stampfile as the prerequisite
for the toolchain/compile target stampfile.

The previous filename was also incorrect,
the stampfile name format was changed (see reference)
but was never updated here.

The issue was never noticed
because the usage of timestamp.pl was broken
(fixed in previous commit)
causing the extra prereq files passed to 'call stampfile'
to never get checked at all.

Fixes: 5149ed151 ("gcc: split up the build process into three distinct stages")
Signed-off-by: Michael Pratt <mcpratt@pm.me>